feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/fixed/src/impl_bytemuck.rs

@@ -0,0 +1,66 @@
+// Copyright © 2018–2025 Trevor Spiteri
+
+// This library is free software: you can redistribute it and/or
+// modify it under the terms of either
+//
+//   * the Apache License, Version 2.0 or
+//   * the MIT License
+//
+// at your option.
+//
+// You should have recieved copies of the Apache License and the MIT
+// License along with the library. If not, see
+// <https://www.apache.org/licenses/LICENSE-2.0> and
+// <https://opensource.org/licenses/MIT>.
+
+use crate::types::extra::{LeEqU8, LeEqU16, LeEqU32, LeEqU64, LeEqU128};
+use crate::{
+    FixedI8, FixedI16, FixedI32, FixedI64, FixedI128, FixedU8, FixedU16, FixedU32, FixedU64,
+    FixedU128, Unwrapped, Wrapping,
+};
+use bytemuck::{Contiguous, Pod, TransparentWrapper, Zeroable};
+
+macro_rules! unsafe_impl_traits {
+    ($Fixed:ident, $LeEqU:ident, $Inner:ident) => {
+        unsafe impl<Frac> Zeroable for $Fixed<Frac> {}
+        unsafe impl<Frac: 'static> Pod for $Fixed<Frac> {}
+        unsafe impl<Frac: 'static> Contiguous for $Fixed<Frac> {
+            type Int = $Inner;
+            const MAX_VALUE: $Inner = $Inner::MAX;
+            const MIN_VALUE: $Inner = $Inner::MIN;
+        }
+        unsafe impl<Frac> TransparentWrapper<$Inner> for $Fixed<Frac> {}
+
+        unsafe impl<Frac: $LeEqU> Zeroable for Wrapping<$Fixed<Frac>> {}
+        unsafe impl<Frac: $LeEqU> Pod for Wrapping<$Fixed<Frac>> {}
+        unsafe impl<Frac: $LeEqU> Contiguous for Wrapping<$Fixed<Frac>> {
+            type Int = $Inner;
+            const MAX_VALUE: $Inner = $Inner::MAX;
+            const MIN_VALUE: $Inner = $Inner::MIN;
+        }
+        unsafe impl<Frac: $LeEqU> TransparentWrapper<$Fixed<Frac>> for Wrapping<$Fixed<Frac>> {}
+
+        unsafe impl<Frac: $LeEqU> Zeroable for Unwrapped<$Fixed<Frac>> {}
+        unsafe impl<Frac: $LeEqU> Pod for Unwrapped<$Fixed<Frac>> {}
+        unsafe impl<Frac: $LeEqU> Contiguous for Unwrapped<$Fixed<Frac>> {
+            type Int = $Inner;
+            const MAX_VALUE: $Inner = $Inner::MAX;
+            const MIN_VALUE: $Inner = $Inner::MIN;
+        }
+        unsafe impl<Frac: $LeEqU> TransparentWrapper<$Fixed<Frac>> for Unwrapped<$Fixed<Frac>> {}
+    };
+}
+
+// SAFETY: all fixed-point numbers are repr(transparent) over primitive integer
+// types which are both Pod and Zeroable, and Wrapping and Unwrapped are both
+// repr(transparent) over fixed-point numbers.
+unsafe_impl_traits! { FixedI8, LeEqU8, i8 }
+unsafe_impl_traits! { FixedI16, LeEqU16, i16 }
+unsafe_impl_traits! { FixedI32, LeEqU32, i32 }
+unsafe_impl_traits! { FixedI64, LeEqU64, i64 }
+unsafe_impl_traits! { FixedI128, LeEqU128, i128 }
+unsafe_impl_traits! { FixedU8, LeEqU8, u8 }
+unsafe_impl_traits! { FixedU16, LeEqU16, u16 }
+unsafe_impl_traits! { FixedU32, LeEqU32, u32 }
+unsafe_impl_traits! { FixedU64, LeEqU64, u64 }
+unsafe_impl_traits! { FixedU128, LeEqU128, u128 }